1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device having floating gate type MOSFETs as memory cells.
2. Description of the Related Art
Conventionally, EEPROM and UVEPROM are known as the nonvolatile semiconductor memory device with a floating gate structure, for example. In the EEPROM, data is electrically written in or programmed and electrically erased. With a memory cell in the EEPROM, data can be programmed by injecting or emitting electrons into or from the floating gate via an oxide film with a thickness of approx. 100 .ANG. which is extremely thinner than a gate oxide film by use of the tunnel effect. The EEPROM is explained in detail in U.S. Pat. No. 4,203,158 (Frohman-Bentchkowsky et al. "ELECTRICALLY PROGRAMMABLE AND ERASABLE MOS FLOATING GATE MEMORY DEVICE EMPLOYING TUNNELING AND METHOD OF FABRICATING SAME").
However, since, in the above EEPROM, two transistors are used to constitute a single memory cell, the memory cell size becomes large and the chip cost will increase.
For the above reason, ultraviolet erasable nonvolatile semiconductor memory devices or UVEPROM has an advantage in attaining high integration density, in which each memory cell is formed of a single transistor. In the UVEPROM, data can be electrically programmed and erased by applying ultraviolet rays thereto. As described above, in the UVEPROM, each memory cell is formed of a single transistor so that the chip size can be reduced for the same memory scale or capacity as that of the EEPROM.
However, in the UVEPROM, a high power source voltage is required to program data. That is, in order to inject electrons into the floating gate of a selected memory cell, a high voltage is applied between the control gate and drain to cause impact ionization in an area near the drain region, injecting the electrons thus generated into the floating gate. For this purpose, it becomes necessary to provide a power source of high voltage for data programming outside the memory device. In contrast, since electrons are injected into or emitted from the floating gate by the tunnel effect in the EEPROM, it is not necessary to use such a programming power source as is used in the UVEPROM and data can be programmed by an output voltage of a booster circuit provided in the same chip as that of the memory device. Therefore, the EEPROM can be operated on a single power source voltage of 5 V.
As described above, the UVEPROM can be formed at a higher integration density in comparison with the EEPROM. However, in general, since a single contact portion is formed for each common drain of two memory cell transistors, the number of contact portions increases. Increase in the number of contact portions is an obstruction to the attainment of high integration and large memory capacity. For this reason, the UVEPROM can be formed at a higher integration density than the EEPROM, but can be further improved in its integration density.